Field of the Invention
Embodiments of the present invention generally relate to diagnostic imaging systems. More particularly, the present invention relates to a method for performing emission computed tomography (“ECT”) scanning, including positron emission tomography (PET) scanning and single photon emission tomography (SPECT). Embodiments of the present invention also provide for improved methods of gating medical images.
Description of the Related Art
Computed tomography (“CT”) scanning (i.e., using an external X-ray source) and positron emission tomography (“PET”) scanning using an infused radiopharmaceutical as a source of gamma ray emissions) are well known methods for diagnostic medical imaging. CT scanning employs multiple X-ray images taken in multiple directions to generate a 3-dimensional image or multiple tomographic image “slices.” PET scanning employs a gamma-emitting radiopharmaceutical ingested by a patient or injected into a patient. Multiple gamma ray images are taken in multiple directions to generate a 3-dimensional PET image or multiple slices.
PET scanning requires a relatively long duration data acquisition period lasting several minutes per patient bed position. Typically, a large number of PET data acquisitions are acquired at many different angles during this period. Consequently, patient movement is a problem in PET scanning Excessive motion of a patient can result in reduced image fidelity, including an incorrect impression of the pattern of tracer uptake, and quantitative errors in which the wrong estimate of tracer concentration is made. Thoracic cage movement caused by breathing is a significant problem in PET scanning.
By comparison, CT scanning is relatively fast and can typically be performed during one breath-hold by a patient.
Part of the solution to the problem of respiration related image degradation is to provide gating of PET scanning based on measurement of certain triggering parameters associated with respiratory motion. In particular, it is known in the art to use a strain gauge to measure the tension in a strap placed around the abdomen or chest of a patient. The time-varying strain measurement is interpreted as a measure of respiratory amplitude and as such is used to develop information that can be used to gate or trigger the operation of imaging apparatus.
In the state of the art, the respiratory amplitudes are used to generate trigger signals, or gates, which indicate that a particular phase in the respiratory cycle has been reached. Commonly, triggers are generated at the end of each full breath, or end-inspiration, and the phase angle is assumed to vary smoothly from trigger to trigger. However, the phase-based approach has a limited ability to identify the actual state of breathing, since patient breathing patterns change over the time period involved in performing the diagnostic scan. This problem is illustrated in the strain gauge traces of FIG. 1. The traces show that deep, irregular breathing at one point in time can be followed by a more regular, shallower breathing pattern ten minutes later. In FIG. 1, the horizontal axis represents time, with a one minute interval between the left and right sides of each plot. The vertical axis represents the strain measurement value. Smaller values correspond to a more relaxed chest or shallow breathing. Larger values correspond to a more expanded chest or deep breathing.
Accordingly, there is a need in the art for improved methods for gating of medical images. It would be particularly beneficial to provide methods of creating medical images that can correct for inaccuracies caused by respiration.